Charged porous polymeric membrane with high void volume

ABSTRACT

Membranes comprising a single layer having a first microporous surface; a second microporous surface; and, a porous bulk between the first microporous surface and the second microporous surface, wherein the bulk comprises a first set of pores having outer rims, prepared by removing introduced dissolvable silica nanoparticles, the first set of pores having a first controlled pore size, and a second set of pores connecting the outer rims of the first set of pores, the second set of pores having a second controlled pore size, and a polymer matrix supporting the first set of pores, wherein the first controlled pore size is greater than the second controlled pore size, and wherein the first and/or second microporous surface comprises a neutrally charged, a negatively charged, or a positively charged, surface; filters including the membranes, and methods of making and using the membranes, are disclosed.

BACKGROUND OF THE INVENTION

Polymeric membranes are used to filter a variety of fluids. However,there is a need for membranes that provide high throughput performance.

The present invention provides for ameliorating at least some of thedisadvantages of the prior art. These and other advantages of thepresent invention will be apparent from the description as set forthbelow.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the invention provides a membrane, comprising a singlelayer having a first microporous surface; a second microporous surface;and, a porous bulk between the first microporous surface and the secondmicroporous surface, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvable silcananoparticles, the first set of pores having a first controlled poresize, and a second set of pores connecting the outer rims of the firstset of pores, the second set of pores having a second controlled poresize, and a polymer matrix supporting the first set of pores, whereinthe first controlled pore size is greater than the second controlledpore size, wherein the first and/or second microporous surface comprisesa neutrally charged, a negatively charged, or a positively charged,microporous surface.

In an embodiment of the membrane, the porous bulk comprises a neutrallycharged bulk, a negatively charged bulk and/or a positively chargedbulk. In some embodiments, the membrane comprises at least one zonecomprising the porous bulk, wherein the zone comprises a neutral charge,a negative charge and/or a positive charge.

In some embodiments, the membrane has at least one additional porousbulk, wherein the additional bulk comprises a third set of pores havingouter rims, prepared by removing introduced dissolvable silicananoparticles, the third set of pores having a third controlled poresize, and a fourth set of pores connecting the outer rims of the thirdset of pores, the fourth set of pores having a fourth controlled poresize, and a polymer matrix supporting the third set of pores, whereinthe third controlled pore size is greater than the fourth controlledpore size. The at least one additional bulk can comprise a neutrallycharged zone, a negatively charged zone and/or a positively chargedzone.

Additionally, or alternatively, the membrane can further comprise atleast one additional layer, wherein the additional layer(s) can comprisea neutrally charged, a negatively charged, or a positively charged,microporous surface (or porous surface), in some embodiments, alsocomprising a neutrally charged, a negatively charged, or a positivelycharged, bulk.

In accordance with other embodiments of the invention, filters andfilter devices comprising the membranes, as well of methods of makingand using the membranes, are provided.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a scanning electron micrograph (SEM) showing a surface view ofan embodiment of a membrane according to the present invention, showinga first set of pores having connecting outer rims (one pore highlightedin dashed lines), and a second set of pores (one pore highlighted insolid line) located in the connecting outer rims of the first set ofpores.

FIG. 2 illustrates hexagonal packing of the first set of pores (formedby dissolving of particles) in a membrane according to an embodiment ofthe invention, wherein the hexagonal packing is 74 volume percent. FIG.2 also illustrates the matrix (“polymer formed interstitials”)supporting the first set of pores, and the second set of poresconnecting the outer rims of the first set of pores.

FIG. 3 is a diagrammatic representation of an embodiment of a membraneaccording to an embodiment of the invention, comprising a first zonecomprising a first porous bulk and a second zone comprising a secondporous bulk, wherein the first zone comprises a negative charge, and thesecond zone comprises a positive charge.

FIG. 4 is an SEM showing an enlarged partial cross-sectional view of amembrane according to an embodiment of the present invention, showing afirst zone comprising a first porous bulk and a second zone comprising asecond porous bulk, wherein the first zone comprises a negative charge,and the second zone comprises a positive charge.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an embodiment of the present invention, a membrane isprovided, the membrane comprising a single layer having a firstmicroporous surface; a second microporous surface; and, a porous bulkbetween the first microporous surface and the second microporoussurface, wherein the bulk comprises a first set of pores having outerrims, prepared by removing introduced dissolvable silica nanoparticles,the first set of pores having a first controlled pore size, and a secondset of pores connecting the outer rims of the first set of pores, thesecond set of pores having a second controlled pore size, and a polymermatrix supporting the first set of pores, wherein the first controlledpore size is greater than the second controlled pore size, wherein thefirst and/or second microporous surface comprises a neutrally chargedsurface, a negatively charged surface, or a positively charged surface.

In an embodiment of the membrane, the porous bulk comprises a neutrallycharged bulk, a negatively charged bulk and/or a positively chargedbulk. In some embodiments, the membrane comprises at least one zonecomprising the porous bulk, wherein the zone comprises a neutral charge,a negative charge and/or a positive charge.

In some embodiments, the membrane has at least one additional porousbulk (e.g., as part of an additional layer or region of the membrane),wherein the additional bulk comprises a third set of pores having outerrims, prepared by removing introduced dissolvable silica nanoparticles,the third set of pores having a third controlled pore size, and a fourthset of pores connecting the outer rims of the third set of pores, thefourth set of pores having a fourth controlled pore size, and a polymermatrix supporting the third set of pores, wherein the third controlledpore size is greater than the fourth controlled pore size. The at leastone additional bulk can comprise at least one neutrally charged zone,negatively charged zone and/or a positively charged zone and/or aneutrally charged zone.

Alternatively, or additionally, in some embodiments, the membranefurther comprises at least one additional layer, the layer having (i) afirst porous surface; (ii) a second porous surface; and, (iii) a porousbulk between the first porous surface and the second porous surface ofthe additional layer, wherein the porous bulk comprises a fibrousmatrix; or (iv) a first microporous surface; (v) a second microporoussurface; and, (vi) a porous bulk between the first microporous surfaceand the second microporous surface of the additional layer, wherein thesecond porous bulk comprises: (a) a first set of pores having outerrims, prepared by removing introduced dissolvable silica nanoparticles,the first set of pores having a first controlled pore size, and a secondset of pores connecting the outer rims of the first set of pores, thesecond set of pores having a second controlled pore size, and a polymermatrix supporting the first set of pores, wherein the first controlledpore size is greater than the second controlled pore size; (b) a set ofpores prepared by phase inversion, the set of pores having a controlledpore size; or (c) a fibrous matrix; or, (d) a set of pores prepared bystretching or track etching or e-beam, the set of pores having acontrolled pore size. In an embodiment, the additional layer comprisesat least one neutrally charged zone, negatively charged zone and/orpositively charged surface, in some embodiments, also comprising atleast one neutrally charged zone, negatively charged zone and/orpositively charged zone.

In some embodiments membranes according to the invention are integral(i.e., the layers, zones and/or regions are bonded together such thatthe membrane behaves as a single structure that does not delaminate orseparate under normal use conditions). For example, while making themembranes, portions of each layer, zone, and/or region can infiltrateeach other and mix.

In accordance with an embodiment, the controlled pore size of the firstset of pores (and/or the controlled pore size of another set of poresprepared by removing introduced silica nanoparticles) is in the range offrom about 50 nm to about 1000 nm, for example, from about 160 nm toabout 630 nm. Thus, for example, the pore size of the first set of poresis about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm,about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm,about 360 nm, about 380 nm, about 400 nm, about 420 nm, about 440 nm,about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm,about 560 nm, about 580 nm, about 600 nm, or about 620 nm.

In an embodiment, the second controlled pore size in the porous bulk (orthe controlled pore size in any other bulk with respect to the poresconnecting pores having outer rims) is in a ratio in the range of about0.2 to about 0.4 times the first controlled pore size (or the controlledpore size with respect to pores having outer rims).

In some embodiments, the controlled pore size of pores having outer rimsin one porous bulk is greater than the controlled pore size of pores inanother bulk or layer, e.g., wherein the controlled pore size of poresin another bulk or layer comprises the retentive portion of thecomposite membrane. In some other embodiments, the controlled pore sizeof pores having outer rims in the bulk is less than the controlled poresize of pores in another bulk or layer, e.g., wherein bulk comprisingpores having outer rims comprises the retentive portion of the compositemembrane.

In an embodiment, the membrane is prepared by introducing dissolvablesilica nanoparticles into solutions comprising one or more membraneforming polymers and charged polymers (typically, the membrane formingpolymers and charged polymers are dissolved in a solvent or mixture ofsolvents), casting the nanoparticle-containing polymer solution(preferably, casting the nanoparticle-containing polymer solution on asubstrate wherein the substrate has been pretreated with apreconditioning or releasing agent; more preferably, wherein the agenthas been dried on the substrate before casting the solution thereon),carrying out phase inversion of the nanoparticle-containing polymersolution to provide a membrane comprising a neutrally charged,negatively charged or positively charged surface, subsequentlydissolving the nanoparticles, and washing the resultant membrane.

The charged polymers are polymers that provide a positive or negativecharge. Positively charged polymers can be any polymer that carries apositive charge or can become positively charged when exposed to fluids.Thus, for example, polymers bearing amino, imino, ammonium, orphosphonium groups in the backbone, pendant groups, and/or on the chainends are suitable. Examples of polymers having positive charge includepolyethylenimine, polydiallyldimethylammonium chloride, polyvinylamine,amine terminated polyethylene oxide or polyethylene glycol,poly(2-vinylpyridine), poly(4-vinylpyridine), andpoly(1-vinylpyrrolidone-co-2-dimethylaminoethylmethacrylate).

Negatively charged polymers can be any polymer that carries a negativecharge or can become negatively charged when exposed to fluids. Thus,for example, polymers bearing carboxyl, sulfonic, or phosphonic groupsin the backbone, pendant groups, and/or chain ends are suitable.Examples of polymers having negative charge include maleic acid:methylvinyl ether copolymer, polystyrene sulfonic acid, sulfonated polysufone,sulfonated polyether sulfone, polyacrylic acid, polymethacrylic acid,and polyvinylphenol.

Advantageously, membranes according to the invention can be producedusing preformed polymers such as polyethersulfone (PES), polyvinylidenefluoride (PVDF), and polyacrylonitrile (PAN), that are commonly used incommercial membranes. Additionally, the nanoparticles can be dissolvedwithout using hydrofluoric acid, for example, the nanoparticles can bedissolved using safer, more environmentally benign solvents.

In other embodiments, filters and filter devices are provided, thefilter and filter devices comprising at least one membrane.

A method of filtering fluid is also provided in accordance with anotherembodiment of the invention, the method comprising passing the fluidthrough at least one membrane, or a filter comprising at least onemembrane, as described above.

In accordance with the invention, a charged zone (sometimes referred tobelow as a “charged continuous zone”) refers to a charge localized in agenerally predetermined portion of the thickness of the membrane,generally parallel to the major surfaces (upstream and downstreamsurfaces) of the membrane. In contrast, mosaic membranes have aplurality of separate (non-continuous) anion-exchange regions,cation-exchange regions, and neutral regions throughout the membrane.

Embodiments of membranes according to the invention include a variety ofcharged groups, and suitable chemistries for providing membranes withcharged groups are known in the art.

If desired, there are a number of procedures for confirming the presenceof the desired charges in the surfaces, regions, and/or zones in themembranes. For example, dye screening tests can be carried out usingcharged dyes, for example, positively charged dyes such as methyleneblue or toluidine blue, and negatively charged dyes such as metanilyellow or Ponceau S red. Alternatively, or additionally, the zetapotentials can be determined, e.g., by determining streaming potentialsat various pHs.

In accordance with an embodiment of the invention, a method of preparinga membrane comprises (a) casting a solution comprising a dissolvablesilica nanoparticle-containing polymer solution comprising an unchargedpolymer or charged polymer onto a substrate; (b) carrying out phaseinversion of the nanoparticle-containing polymer solution to provide amembrane; (c) dissolving the nanoparticles and obtaining ananoparticle-depleted membrane comprising a negatively charged orpositively charged surface; and (d) washing the nanoparticle-depletedmembrane.

Preferably (a) comprises casting the solution on a substrate pretreatedwith a preconditioning agent or a release agent. In some embodiments ofthe method, the preconditioning agent or the release agent is dried onthe substrate before casting the solution on the pretreated substrate.

In some embodiments, the method comprises exposing thenanoparticle-containing polymer solution to a temperature in the rangeof from about 40° C. to about 80° C. for a period in the range of fromabout 1 minute to about 2 hours. Alternatively, or additionally, in someembodiments, the method comprises forming a film, and immersing the filmin liquid to obtain the membrane.

As will be described in more detail below, dissolving the particlescreates a first set of pores in the membranes, the first set of poreshaving outer rims, and located within the outer rims is a second set ofpores. As illustrated in FIG. 1, the dashed line highlights an outer rimof a pore in the first set, and the solid line highlights a pore in thesecond set. The second set of pores allows communication (e.g., fluidflow) from the void within one outer rim into the void of another outerrim.

A variety of dissolvable silica nanoparticles are suitable for use inpreparing membranes according to embodiments of the invention.Preferably, the dissolvable particles are not pure silica. Typically,the particles comprise silica nanoparticles ranging in diameter fromabout 50 nm to about 1000 nm. In an embodiment, the particles comprisesilica nanoparticles ranging in diameter from about 50 nm to about 1000nm, having a density of 1.96 g/cm³ or less. In some embodiments, thesilica nanoparticles have a particle density of about 1.93 to about 1.96g/cm³.

The silica nanoparticles can have a particle size, e.g., diameter, ofless than 1000 nm, in particular a particle size of from about 160 nm toabout 630 nm. Thus, for example, the nanoparticles have a particle sizeof about 160 nm, about 180 nm, about 200 nm, about 220 nm, about 240 nm,about 260 nm, about 280 nm, about 300 nm, about 320 nm, about 340 nm,about 360 nm, about 380 nm, about 400 nm, about 420 nm, about 440 nm,about 460 nm, about 480 nm, about 500 nm, about 520 nm, about 540 nm,about 560 nm, about 580 nm, about 600 nm, or about 620 nm.

The silica nanoparticles can be prepared by a method comprising: (a)reacting an orthosilicate and an alcohol or a mixture of alcohols in anaqueous medium in the presence of a salt of a metal of Group Ia or GroupIIa, or in the presence of a metalloid compound, optionally incombination with ammonium hydroxide, (b) isolating the resultingnanoparticles, and (c) treating the nanoparticles from (b) with an acid.

In an embodiment, the nanoparticles can be included in the coatingcomposition prior to the acid treatment (c).

In an embodiment, the orthosilicate used in the preparation of thenanoparticles is a tetraalkylorthosilicate. Examples oftetraalkylorthosilicates tetramethylorthosilicate,tetraethylorthosilicate, tetrapropylorthosilicate,tetrabutylorthosilicate, and tetrapentylorthosilicate.

Any suitable alcohol or mixture of alcohols can be used in thepreparation of the nanoparticles, for example, the alcohol or mixture ofalcohols is selected from methanol, ethanol, propanol, butanol, andmixtures thereof.

The salt of the metal used in the preparation of the nanoparticles canbe selected from salts of lithium, sodium, potassium, cesium, magnesium,and calcium. In an embodiment, the salt of the metal is selected fromlithium acetate, sodium acetate, sodium metasilicate, sodium formate,potassium acetate, cesium acetate, magnesium acetate, and calciumacetate. In another embodiment, the metalloid compound is a compound ofboron, for example, boric acid or a boric acid ester such as alkylborate. The alkyl borate can be a trialkyl borate such as trimethylborate or triethyl borate.

The acid employed in (c) of the method above can be a mineral acid ororganic acid. Examples of mineral acids include hydrochloric acid,sulfuric acid, and nitric acid, preferably hydrochloric acid or sulfuricacid. Examples of organic acids include acetic acid, formic acid,trifluoroacetic acid, trichloroacetic acid, and p-toluenesulfonic acid,preferably formic acid. The nanoparticles isolated in (b) can be treatedwith a 1N to 2N acid, e.g., 1N HCl, or 10-50% weight % organic acid inwater, e.g., 50% aqueous formic acid, for a period of about 0.5 hr toabout 3 hr, preferably about 1 hr to 2 hr. For example, thenanoparticles can be sonicated in an acid bath for the above period.Following the acid treatment, the nanoparticles are isolated from theacid and washed with deionized water and dried under vacuum to obtainthe silica nanoparticles.

Illustratively, silica nanoparticles can be prepared as follows. In a 6L jacketed flask kept at 25° C., 4.8 g lithium acetate dihydrate(LiOAc.2H₂O), 2480 mL deionized water (DI-H₂O), 2.9 L anhydrous ethanol(EtOH), and 120 mL 28% w/w NH₃ in water is stirred for 30 min at 200 rpmusing an overhead mixer with PTFE impellers. A solution of 300 mL EtOHwith 200 mL tetraethylorthosilicate (TEOS), which is prepared under dryconditions (<10% relative humidity), is rapidly poured into the 6 Lflask, and mixing is increased to 400 rpm and a dry air purge (<1%relative humidity) is utilized for 5 min. Mixing is reduced to 200 rpm,the dry air purge is removed, the flask is sealed, and the reactioncontinues for a total of 1 h. The particles are purified bycentrifugation and re-suspension in EtOH three times.

Typical stock solutions comprising the dissolvable nanoparticles,preferably purified dissolvable nanoparticles, comprise thenanoparticles dispersed at concentrations in the range of from about 30wt % to about 65 wt % dimethyl formamide (DMF), with in the range offrom about 0.001% to about 0.1% triethanolamine (TEA).

A variety of procedures are suitable for dissolving the particles. Asnoted above, the process should avoid using hydrofluoric acid; rather,the nanoparticles can be, and should be, dissolved using safer, moreenvironmentally benign solvents. For example, thenanoparticle-containing membrane can be placed in a mineral acid (e.g.,HCl or H₂SO₄) at a concentration in the range of about 0.1 to about 2moles/L for a period in the range of from about 1 minute to about 1hour, followed by immersion in an alkaline solution (e.g., KOH or NaOH)at a concentration in the range of about 0.1 to about 4 moles/L for aperiod in the range of from about 30 minutes to about 24 hours, followedby washing in water (e.g., DI water) for a period in the range of about30 minutes to about 4 hours. If desired, the membrane can subsequentlybe dried, e.g., in an oven at a temperature in the range of from about40° C. to about 80° C. for a period in the range of about 30 minutes toabout 2 hours.

Typically, the phase inversion process for producing the membrane fromthe nanoparticle-containing polymer solution involves casting orextruding a polymer solution into a thin film on the substrate, andprecipitating the polymer(s) through one or more of the following: (a)evaporation of the solvent and nonsolvent, (b) exposure to a non-solventvapor, such as water vapor, which absorbs on the exposed surface, (c)quenching in a non-solvent liquid (e.g., a phase immersion bathcontaining water, and/or another non-solvent or solvent), and (d)thermally quenching a hot film so that the solubility of the polymer issuddenly greatly reduced. Phase inversion can be induced by the wetprocess (immersion precipitation), vapor induced phase separation(VIPS), thermally induced phase separation (TIPS), quenching, dry-wetcasting, and solvent evaporation (dry casting). Dry phase inversiondiffers from the wet or dry-wet procedure by the absence of immersioncoagulation. In these techniques, an initially homogeneous polymersolution becomes thermodynamically unstable due to different externaleffects, and induces phase separation into a polymer lean phase and apolymer rich phase. The polymer rich phase forms the matrix of themembrane, and the polymer lean phase, having increased levels ofsolvents and non-solvents, forms pores.

A membrane-forming polymer solution is prepared by dissolving thepolymer in a solvent or a mixture of solvents. A variety of polymersolutions are suitable for use in the invention, and are known in theart. Suitable polymer solutions can include, polymers such as, forexample, polyaromatics; sulfones (e.g., polysulfones, including aromaticpolysulfones such as, for example, polyethersulfone (PES), polyetherether sulfone, bisphenol A polysulfone, polyarylsulfone, andpolyphenylsulfone), polyamides, polyimides, polyvinylidene halides(including polyvinylidene fluoride (PVDF)), polyolefins, such aspolypropylene and polymethylpentene, polyesters, polystyrenes,polycarbonates, polyacrylonitriles ((PANs) includingpolyalkylacrylonitriles), cellulosic polymers (such as celluloseacetates and cellulose nitrates), fluoropolymers, and polyetheretherketone (PEEK). Polymer solutions can include a mixture of polymers,e.g., a hydrophobic polymer (e.g., a sulfone polymer) and a hydrophilicpolymer (e.g., polyvinylpyrrolidone (PVP)).

In addition to one or more polymers, typical polymer solutions compriseat least one solvent, and may further comprise at least one non-solvent.Suitable solvents include, for example, dimethyl formamide (DMF);N,N-dimethylacetamide (DMAC); N-methyl pyrrolidone (NMP); dimethylsulfoxide (DMSO), methyl sulfoxide, tetramethylurea; dioxane; diethylsuccinate; chloroform; and tetrachloroethane; and mixtures thereof.Suitable nonsolvents include, for example, water; various polyethyleneglycols (PEGs; e.g., PEG-200, PEG-300, PEG-400, PEG-1000); variouspolypropylene glycols; various alcohols, e.g., methanol, ethanol,isopropyl alcohol (IPA), amyl alcohols, hexanols, heptanols, andoctanols; alkanes, such as hexane, propane, nitropropane, heptanes, andoctane; and ketone, ethers and esters such as acetone, butyl ether,ethyl acetate, and amyl acetate; acids, such as acetic acid, citricacid, and lactic acid; and various salts, such as calcium chloride,magnesium chloride, and lithium chloride; and mixtures thereof.

If desired, a solution comprising a polymer can further comprise, forexample, one or more polymerization initiators (e.g., any one or more ofperoxides, ammonium persulfate, aliphatic azo compounds (e.g.,2,2'-azobis(2-amidinopropane) dihydrochloride (V50)), and combinationsthereof), and/or minor ingredients such as surfactants and/or releaseagents.

Typical stock solutions including a polymer (before combining with asolution comprising the dissolvable nanoparticles) comprise in the rangeof from about 10 wt % to about 35 wt % resin (e.g., PES, PVDF, or PAN),in the range of from about 0 to about 10 wt % PVP, in the range of fromabout 0 to about 10 wt % PEG, in the range of from about 0 to about 90wt % NMP, in the range of from about 0 to about 90 wt % DMF, and in therange of from about 0 to about 90 wt % DMAC. When a charged polymer isincluded in the stock solutions, the charged polymer can be present inan amount of up to about 5 wt % relative to total resin only (e.g., ≦5PHR).

Suitable components of solutions are known in the art. Illustrativesolutions comprising polymers, and illustrative solvents andnonsolvents, and illustrative charged groups, include those disclosedin, for example, U.S. Pat. Nos. 4,340,579; 4,629,563; 4,900,449;4,964,990, 5,444,097; 5,846,422; 5,906,742; 5,928,774; 6,045,899;6,146,747; 6,780,327, 6,783,937, 7,208,200, and 7,189,322.

While a variety of polymeric membranes can be produced in accordancewith the invention, in preferred embodiments, the membranes are sulfonemembranes (more preferably, polyethersulfone membranes and/orpolyarylsulfone membranes), acrylic membranes (e.g., (PANs, includingpolyalkylacrylonitriles), or semi-crystalline membranes (for example,PVDF membranes and/or polyamide membranes).

The membranes can be cast manually (e.g., poured, cast, or spread byhand onto the substrate) or automatically (e.g., poured or otherwisecast onto a moving bed having the substrate thereon).

A variety of casting techniques, including multiple casting techniques,are known in the art and are suitable. A variety of devices known in theart can be used for casting. Suitable devices include, for example,mechanical spreaders, that comprise spreading knives, doctor blades, orspray/pressurized systems. One example of a spreading device is anextrusion die or slot coater, comprising a casting chamber into whichthe casting formulation (solution comprising a polymer) can beintroduced and forced out under pressure through a narrow slot.Illustratively, the solutions comprising polymers can be cast by meansof a doctor blade with knife gaps in the range from about 100micrometers to about 500 micrometers, more typically in the range fromabout 120 micrometers to about 400 micrometers.

A variety of casting speeds are suitable as is known in the art.Typically, the casting speed is at least about 3 feet per minute (fpm),more typically in the range of from about 3 to about 40 fpm, in someembodiments, at least about 5 fpm.

A variety of substrates are suitable for preparing membranes accordingto embodiments of the invention. Preferably, the substrate is anon-paper substrate. Suitable substrates include, for example, glass, apolyester such as polyethylene terephthalate (PET) (e.g., commerciallyavailable as MYLAR); polypropylene; polyethylene (including polyethylenenaphthalate (PEN); polyethylene terephthalate glycol (PETG)); polyimide;polyphenylene oxide; nylon; and acrylics.

Also, a variety of media (for use as the second layer, third layer, oradditional layer) as described herein can be used for preparingcomposite membranes according to embodiments of the invention.

In some embodiments, the substrate has been pretreated with apreconditioning agent or release agent, preferably, wherein the agent isdried before the particle-containing polymer solution is cast on thepretreated substrate. Without being bound to any particular theory, itis believed that, with respect to some substrates and/or polymers, theuse of a preconditioning or release agent improves efficiency inseparating the dissolvable particle-containing membrane from thesubstrate, before dissolving the particles.

Preferably, the preconditioning or release agent does not dissolve insolvents used in the casting formulation, is compatible with membraneprocessing temperatures, sufficiently adheres to the cast film duringthermal processing that it does not delaminate, and dissolves readily insolvents that do not dissolve the membrane resin (such that the membranecan be released from the substrate). Examples of suitablepreconditioning or release agents include polyvinyl alcohol (PVOH),polyvinylpyrrolidone (PVP), poly(acrylic acid), and poly(methacrylicacid).

Illustratively, a PVOH stock solution can be prepared with about 5 wt %to about 15 wt % PVOH in deionized water, and cast on a substrate usinga casting bar with a gap in the range of from about 1 to about 10 mil,and dried in an oven at a temperature in the range of from about 40° C.to about 80° C. for a period in the range of from about 1 minute toabout 2 hours.

The membranes can have any suitable pore structure, e.g., a pore size(for example, as evidenced by bubble point, or by K_(L) as described in,for example, U.S. Pat. No. 4,340,479, or evidenced by capillarycondensation flow porometry), a mean flow pore (MFP) size (e.g., whencharacterized using a porometer, for example, a Porvair Porometer(Porvair plc, Norfolk, UK), or a porometer available under the trademarkPOROLUX (Porometer.com; Belgium)), a pore rating, a pore diameter (e.g.,when characterized using the modified OSU F2 test as described in, forexample, U.S. Pat. No. 4,925,572), or removal rating media. The porestructure used depends on the size of the particles to be utilized, thecomposition of the fluid to be treated, and the desired effluent levelof the treated fluid.

Additionally, the membranes have a desirable hexagonal structureresulting from the first set of pores in the bulk of the membrane. Asillustrated in FIG. 2 (showing the first set of pores resulting fromdissolving the introduced particles and the hexagonal structurerepresenting the maximum void fraction), the maximum void fraction is 74volume percent, and membranes according to embodiments of the inventionhave in the range of from about 66% to about 73% void fraction.

The microporous surfaces of the membranes can have any suitable meanpore size, e.g., as determined by, for example, calculating the averagesurface pore size from an SEM at 5,000×or 20,000× magnification.

Typically, the thickness of membranes according to embodiments of theinvention is in the range of about 0.5 mils to about 6.5 mils,preferably, in the range of from about 1 mils to about 3 mils.

In those embodiments with the second, third, and/or additional layercomprises a fibrous matrix, e.g., the layer comprises a fibrous porousmedium (e.g., a fabric web such as woven web or a non-woven web; or awet-laid medium) or comprises a porous membrane made without dissolvableparticles, the medium can be made of any suitable material; preferably,the fibrous medium or membrane comprises a polymeric medium. A varietyof polymers are suitable, and suitable fibrous porous media and porousmembranes without dissolvable particles can be prepared by methods knownto those of ordinary skill in the art.

The membrane can have any desired critical wetting surface tension(CWST, as defined in, for example, U.S. Pat. No. 4,925,572). The CWSTcan be selected as is known in the art, e.g., as additionally disclosedin, for example, U.S. Pat. Nos. 5,152,905, 5,443,743, 5,472,621, and6,074,869. Typically, the membrane has a CWST of greater than about 70dynes/cm (about 70×10⁻⁵ N/cm), more typically greater than about 73dynes/cm (about 73×10⁻⁵ N/cm), and can have a CWST of about 78 dynes/cm(about 78×10⁻⁵ N/cm) or more. In some embodiments, the membrane has aCWST of about 82 dynes/cm (about 82×10⁻⁵N/cm) or more.

The surface characteristics of the membrane can be modified (e.g., toaffect the CWST, to include a surface charge, e.g., a positive ornegative charge, and/or to alter the polarity or hydrophilicity of thesurface) by wet or dry oxidation, by coating or depositing a polymer onthe surface, or by a grafting reaction. Modifications include, e.g.,irradiation, a polar or charged monomer, coating and/or curing thesurface with a charged polymer, and carrying out chemical modificationto attach functional groups on the surface. Grafting reactions may beactivated by exposure to an energy source such as gas plasma, vaporplasma, corona discharge, heat, a Van de Graff generator, ultravioletlight, electron beam, or to various other forms of radiation, or bysurface etching or deposition using a plasma treatment.

A variety of fluids can be filtered in accordance with embodiments ofthe invention. Membranes according to embodiments of the invention canbe used in a variety of applications, including, for example, diagnosticapplications (including, for example, sample preparation and/ordiagnostic lateral flow devices), ink jet applications, filtering fluidsfor the pharmaceutical industry, filtering fluids for medicalapplications (including for home and/or for patient use, e.g.,intravenous applications, also including, for example, filteringbiological fluids such as blood (e.g., to remove leukocytes)), filteringfluids for the electronics industry (e.g., filtering photoresist fluidsin the microelectronics industry), filtering fluids for the food andbeverage industry, clarification, filtering antibody-and/orprotein-containing fluids, filtering nucleic acid-containing fluids,cell detection (including in situ), cell harvesting, and/or filteringcell culture fluids. Alternatively, or additionally, membranes accordingto embodiments of the invention can be used to filter air and/or gasand/or can be used for venting applications (e.g., allowing air and/orgas, but not liquid, to pass therethrough). Membranes according toembodiments of the inventions can be used in a variety of devices,including surgical devices and products, such as, for example,ophthalmic surgical products.

In use filtering a fluid, the membrane zones, regions, and/or layers inthe composite membrane can be in any order, depending on theapplication. For example, the first layer, zone, or region, or any otherlayer, zone, or region can be the “upstream” layer, zone, or region,i.e., the layer, zone, or region first contacted by the fluid.Alternatively, the first layer, zone, or region, or any other layer,zone, or region, can be the “downstream” layer, zone, or region, i.e.,the layer , zone, or region last contacted by the fluid. In yet anotheralternative, in those embodiments having at least three layers, zones,or regions, any layer, zone, or region can be the “upstream,”“downstream,” or an intermediate layer, zone, or region.

In one embodiment wherein the upstream portion of the membrane comprisesa positively charged zone and the downstream portion of the membranecomprises a negatively charged zone, a method of treating fluidcomprises passing a fluid comprising a higher concentration of anionsthan cations from the upstream surface of the membrane through thedownstream surface. Without being limited to any particular mechanism,it is believed that as the anions are captured and/or bound in thepositively charged zone, the anions can repulse some additional anions,providing a prefiltration function, and allowing more of the positivelycharged zone to be available for further adsorption to capacity.

Similarly, in one embodiment wherein the upstream portion of themembrane comprises a negatively charged zone and the downstream portionof the membrane comprises a positively charged zone, a method oftreating fluid comprises passing a fluid comprising a higherconcentration of cations than anions from the upstream surface of themembrane through the downstream surface. Without being limited to anyparticular mechanism, it is believed that as the cations are capturedand/or bound in the negatively charged zone, the cations can repulsesome additional cations, providing a prefiltration function, andallowing more of the negatively charged zone to be available for furtheradsorption to capacity.

In accordance with embodiments of the invention, the membrane can have avariety of configurations, including planar, pleated, and/or hollowcylindrical.

Membranes according to embodiments of the invention are typicallydisposed in a housing comprising at least one inlet and at least oneoutlet and defining at least one fluid flow path between the inlet andthe outlet, wherein at least one inventive membrane or a filterincluding at least one inventive membrane is across the fluid flow path,to provide a filter device or filter module. In an embodiment, a filterdevice is provided comprising a housing comprising an inlet and a firstoutlet, and defining a first fluid flow path between the inlet and thefirst outlet; and at least one inventive membrane or a filter comprisingat least one inventive membrane, the inventive membrane or filtercomprising at least one inventive membrane being disposed in the housingacross the first fluid flow path.

Preferably, for crossflow applications, at least one inventive membraneor filter comprising at least one inventive membrane is disposed in ahousing comprising at least one inlet and at least two outlets anddefining at least a first fluid flow path between the inlet and thefirst outlet, and a second fluid flow path between the inlet and thesecond outlet, wherein the inventive membrane or filter comprising atleast one inventive membrane is across the first fluid flow path, toprovide a filter device or filter module. In an illustrative embodiment,the filter device comprises a crossflow filter module, the housingcomprising an inlet, a first outlet comprising a concentrate outlet, anda second outlet comprising a permeate outlet, and defining a first fluidflow path between the inlet and the first outlet, and a second fluidflow path between the inlet and the second outlet, wherein at least oneinventive membrane or filter comprising at least one inventive membraneis disposed across the first fluid flow path.

The filter device or module may be sterilizable. Any housing of suitableshape and providing an inlet and one or more outlets may be employed.

The housing can be fabricated from any suitable rigid imperviousmaterial, including any impervious thermoplastic material, which iscompatible with the fluid being processed. For example, the housing canbe fabricated from a metal, such as stainless steel, or from a polymer,e.g., transparent or translucent polymer, such as an acrylic,polypropylene, polystyrene, or a polycarbonated resin.

The following examples further illustrate the invention but, of course,should not be construed as in any way limiting its scope.

In the following examples, dissolvable particles are prepared in stocksolutions, and PVOH stock solution are prepared, as described below.

Stock solutions having nanoparticles having diameters of approximately570 nm are prepared as follows: Dissolvable particles are prepared instock solution as follows: In a jacketed flask kept at 25° C., asolution is prepared consisting of 1 mol/L ammonia (NH₃), 8.24 mol/Lethanol (ETOH), 1 mol/L methanol (MeOH), 23.7 mol/L deionized (DI)water, 0.15 mol/L tetraethoxysilane (TEOS), and 0.0078 mol/L sodiummetasilicate (Na₂SiO₃), and stirred at 200 rpm for 1 hr. Dynamic lightscattering and SEM show particle diameters of approximately 570 nm.Particles are centrifuged, decanted, and re-suspended in ETOH twice.Then, the particles are centrifuged, decanted, and re-suspended in DMFalong with 0.1% triethanolamine (TEA) three times. The stock solutionhas a final concentration of 63% (w/w) particles.

Stock solutions having nanoparticles having diameters of approximately310 nm are prepared as follows: In a jacketed flask kept at 25° C., asolution is prepared consisting of 0.9 mol/L NH₃, 9.16 mol/L ETOH, 23.07mol/L DI water, 0.15 mol/L TEOS, and 0.0078 mol/L lithium acetate(CH₃COOLi), and stirred at 200 rpm for 1 hr. Dynamic light scatteringand SEM show particle diameters of approximately 310 nm. Particles arecentrifuged, decanted, and re-suspended in ETOH twice. Then, theparticles are centrifuged, decanted, and re-suspended in DMF along with0.1% TEA three times. The stock solution has a final concentration of55% (w/w) particles.

Polyvinyl Alcohol (PVOH) stock solution is prepared as follows: In ajacketed kettle kept at 90° C., a solution is prepared by combining 10%w/w PVOH (96% Hydrolyzed, Scientific Polymer Products) with 90% DI waterand stirring at 200 rpm for 16 hr.

In general, in determining whether a membrane has a positive charge, themembrane is submerged for 30 minutes in a negatively charged dyesolution (Ponceau S (Red) Dye, 0.05% in DI water). The membrane isleached in a 0.1% solution of ammonium hydroxide, followed by DI waterleaching and drying. Digital microscope photos show the red color dye ispresent at the surface and in the bulk of the membrane having thepositive charge, whereas no red color is shown if the membrane has anegative charge, or a neutral charge.

In general, in determining whether a membrane has a negative charge, themembrane is submerged for 30 minutes in a positively charged dyesolution (Toluidine Blue Dye, 0.05% in DI water). The membrane isleached DI water, followed by drying. Digital microscope photos show theblue color dye is present at the surface and in the bulk of the membranehaving the negative charge, whereas no blue color is shown if themembrane has a positive charge, or a neutral charge.

In the following examples, SEM analysis and porometry are used todetermine the second controlled pore size of the second set of pores,that are located in the connections between the outer rims of the firstset of pores.

EXAMPLE 1

This example demonstrates the preparation of a membrane according to anembodiment of the invention, the membrane comprising a single layerhaving first and second microporous surfaces, a porous bulk between thesurfaces, wherein the bulk comprises a first set of pores having outerrims, prepared by removing introduced dissolvable nanoparticles, thefirst set of pores having a first controlled pore size of about 570 nm,and a second set of pores connecting the outer rims of the first set ofpores, the second set of pores having a second controlled pore size ofabout 171 nm, and a polymer matrix supporting the first set of pores,and the surfaces and the bulk have a neutral charge.

A stock solution having dissolvable particles having diameters ofapproximately 570 nm is prepared as described above.

A polymer (resin) stock solution is prepared as follows: In a jacketedkettle kept at 25° C. using a circulating bath, 30% (w/w) PES resin(BASF, Ultrason E 6020 P), 15% (w/w) NMP, and 55% (w/w) DMF are mixed at800 rpm using an overhead mixer for 4 hr. The solution is placed undervacuum at 200 mbar for 30 minutes to deaerate the solution.

A casting solution is prepared as follows: The resin stock solution andthe particle stock solution are combined in a flask and mixed at 30,000rpm for 2 min with final concentrations of 40% (w/w) particles, 11% PES,6% NMP, and 43% DMF, followed by deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the casting solution is cast onto the PVOH film using a 3mil gapped casting bar and placed in an oven for 15 min at 60° C.followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 1 mol/LHCl for 30 min., then 2 mol/L KOH for 18 hr. The membrane issubsequently washed with water at 25° C. for 2 hr and dried at 70° C.for 30 min.

After exposing two separate sections of the membrane individually toPonceau S (Red) Dye, and Toluidine Blue Dye, no prominent red or bluecolor is shown, thus showing the membrane has a neutral charge at thesurface and through the bulk.

EXAMPLE 2

This example demonstrates the preparation of a membrane according toanother embodiment of the invention, the membrane comprising a singlelayer having first and second microporous surfaces, a porous bulkbetween the surfaces, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvablenanoparticles, the first set of pores having a first controlled poresize of about 570 nm, and a second set of pores connecting the outerrims of the first set of pores, the second set of pores having a secondcontrolled pore size of about 171 nm, and a polymer matrix supportingthe first set of pores, and the surfaces and the bulk comprise anegative charge.

A stock solution having dissolvable particles having diameters ofapproximately 570 nm is prepared as described above.

A polymer (resin) stock solution is prepared as follows: In a jacketedkettle kept at 25° C. using a circulating bath, 20% (w/w) PES resin(BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1% PEG-1000,and <1% 1:1 maleic acid:methyl vinyl ether copolymer (Scientific PolymerProducts) are mixed at 800 rpm using an overhead mixer for 4 hr. Thesolution is placed under vacuum at 200 mbar for 30 minutes to deaeratethe solution.

A casting solution is prepared as follows: The resin stock solution andthe particle stock solution are combined in a flask and mixed at 30,000rpm for 2 min with final concentrations of 29% (w/w) particles, 11% PES,11% NMP, 49% DMF, <1% PEG-1000, and <1% 1:1 maleic acid:methyl vinylether copolymer, followed by deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the casting solution is cast onto the PVOH film using a 3mil gapped casting bar and placed in an oven for 15 min at 60° C.followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 1 mol/LHCl for 30 min., then 2 mol/L KOH for 18 hr. The membrane issubsequently washed with water at 25° C. for 2 hr and dried at 70° C.for 30 min.

After exposing the membrane to Toluidine Blue Dye, a prominent bluecolor is shown, thus showing the membrane has a negative charge at thesurfaces and through the bulk.

EXAMPLE 3

This example demonstrates the preparation of a membrane according toanother embodiment of the invention, the membrane comprising a singlelayer having first and second microporous surfaces, a porous bulkbetween the surfaces, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvablenanoparticles, the first set of pores having a first controlled poresize of about 570 nm, and a second set of pores connecting the outerrims of the first set of pores, the second set of pores having a secondcontrolled pore size of about 171 nm, and a polymer matrix supportingthe first set of pores, and the surfaces and the bulk comprise apositive charge.

A stock solution having dissolvable particles having diameters ofapproximately 570 nm is prepared as described above.

A polymer (resin) stock solution is prepared as follows: In a jacketedkettle kept at 25° C. using a circulating bath, 20% (w/w) PES resin(BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1% PEG-1000,and <1% branched polyethylenimine (Sigma-Aldrich) are mixed at 800 rpmusing an overhead mixer for 4 hr. The solution is placed under vacuum at200 mbar for 30 minutes to deaerate the solution.

A casting solution is prepared as follows: The resin stock solution andthe particle stock solution are combined in a flask and mixed at 30,000rpm for 2 min with final concentrations of 29% (w/w) particles, 11% PES,11% NMP, 49% DMF, <1% PEG-1000, and <1% branched polyethylenimine,followed by deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the casting solution is cast onto the PVOH film using a 3mil gapped casting bar and placed in an oven for 15 min at 60° C.followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 1 mol/LHCl for 30 min., then 2 mol/L KOH for 18 hr. The membrane issubsequently washed with water at 25° C. for 2 hr and dried at 70° C.for 30 min.

After exposing the membrane to Ponceau S, a prominent red color isshown, thus showing the membrane has a positive charge at the surfaceand through the bulk.

EXAMPLE 4

This example demonstrates the preparation of a membrane according toanother embodiment of the invention, the membrane comprising a singlelayer having first and second microporous surfaces, a first and secondzones respectively comprising first and second porous bulks between thesurfaces, wherein each of the bulks comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvablenanoparticles, the first set of pores having a first controlled poresize of about 570 nm, and a second set of pores connecting the outerrims of the first set of pores, the second set of pores having a secondcontrolled pore size of about 171 nm, and a polymer matrix supportingthe first set of pores, and wherein one zone comprise a positivelycharged zone and the other zone comprises a separate, negatively chargedzone.

Two stock solution having dissolvable particles having diameters ofapproximately 570 nm is prepared as described above.

A first polymer (resin) stock solution is prepared as follows: In ajacketed kettle kept at 25° C. using a circulating bath, 20% (w/w) PESresin (BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1%PEG-1000, and <1% 1:1 maleic acid:methyl vinyl ether copolymer(Scientific Polymer Products) are mixed at 800 rpm using an overheadmixer for 4 hr. The solution is placed under vacuum at 200 mbar for 30minutes to deaerate the solution.

A first casting solution is prepared as follows: The resin stocksolution and the particle stock solution are combined in a flask andmixed at 30,000 rpm for 2 min with final concentrations of 29% (w/w)particles, 11% PES, 11% NMP, 49% DMF, <1% PEG-1000, and <1% 1:1 maleicacid:methyl vinyl ether copolymer, followed by deaeration at 200 mbarfor 30 min.

A second polymer (resin) stock solution is prepared as follows: In ajacketed kettle kept at 25° C. using a circulating bath, 20% (w/w) PESresin (BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1%PEG-1000, and <1% branched polyethylenimine (Sigma-Aldrich) are mixed at800 rpm using an overhead mixer for 4 hr. The solution is placed undervacuum at 200 mbar for 30 minutes to deaerate the solution.

A second casting solution is prepared as follows: The resin stocksolution and the particle stock solution are combined in a flask andmixed at 30,000 rpm for 2 min with final concentrations of 29% (w/w)particles, 11% PES, 11% NMP, 49% DMF, <1% PEG-1000, and <1% branchedpolyethylenimine, followed by deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the first casting solution is cast onto the PVOH filmusing a 3 mil gapped casting bar and placed in an oven for 15 min at 60°C. Subsequently, the second casting solution is cast onto the coatedusing a 5 mil gapped casting bar and placed in an oven for 15 min at 60°C., followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 1 mol/LHCl for 30 min., then 2 mol/L KOH for 18 hr. The membrane issubsequently washed with water at 25° C. for 2 hr and dried at 70° C.for 30 min.

After exposing the membrane to Ponceau S (Red) Dye, and Toluidine BlueDye, a prominent red color is shown for one zone and free microporoussurface, and a prominent blue color is shown for the other zone and theother free microporous surface, thus showing the membrane has distinctpositively charged and negatively charged zones.

FIG. 3 is a diagrammatic representation of a membrane comprising a firstzone comprising a first porous bulk and a second zone comprising asecond porous bulk, wherein the first zone comprises a negative charge,and the second zone comprises a positive charge.

FIG. 4 is an SEM showing an enlarged partial cross-sectional view of themembrane prepared in the Example, showing the first zone comprising afirst porous bulk and a second zone comprising a second porous bulk,wherein the first zone comprises a negative charge through the bulk, andthe second zone comprises a positive charge through the bulk, alsoshowing that the membrane is integral.

EXAMPLE 5

This example demonstrates the preparation of a membrane according toanother embodiment of the invention, the membrane comprising a singlelayer having first and second microporous surfaces, a porous bulkbetween the surfaces, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvablenanoparticles, the first set of pores having a first controlled poresize of about 310 nm, and a second set of pores connecting the outerrims of the first set of pores, the second set of pores having a secondcontrolled pore size of about 93 nm, and a polymer matrix supporting thefirst set of pores, and the surfaces and the bulk comprise a negativecharge.

A stock solution having dissolvable particles having diameters ofapproximately 310 nm is prepared as described above.

A polymer (resin) stock solution is prepared as follows: In a jacketedkettle kept at 40° C. using a circulating bath, 20% (w/w) PES resin(BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1% PEG-200,and <1% 1:1 poly(methacrylic acid) (Scientific Polymer Products) aremixed at 800 rpm using an overhead mixer for 4 hr. The solution isplaced under vacuum at 200 mbar for 30 minutes to deaerate the solution.

A casting solution is prepared as follows: The resin stock solution andthe particle stock solution are combined in a flask and mixed at 30,000rpm for 2 min with final concentrations of 28% (w/w) particles, 10% PES,10% NMP, 52% DMF, <1% PEG-200, and <1% poly(methacrylic acid), followedby deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the casting solution is cast onto the PVOH film using a 7mil gapped casting bar and placed in an oven for 15 min at 60° C.followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 0.5 mol/LHCl for 30 min., then 2 mol/L KOH for 4 hr. The membrane is subsequentlywashed with water at 25° C. for 2 hr and dried at 70° C. for 30 min.

After exposing the membrane to Toluidine Blue Dye, a prominent bluecolor is shown, thus showing the membrane has a negative charge at thesurfaces and through the bulk.

EXAMPLE 6

This example demonstrates the preparation of a membrane according toanother embodiment of the invention, the membrane comprising a singlelayer having first and second microporous surfaces, a porous bulkbetween the surfaces, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvablenanoparticles, the first set of pores having a first controlled poresize of about 310 nm, and a second set of pores connecting the outerrims of the first set of pores, the second set of pores having a secondcontrolled pore size of about 93 nm, and a polymer matrix supporting thefirst set of pores, and the surfaces and bulk comprise a positivecharge.

A stock solution having dissolvable particles having diameters ofapproximately 310 nm is prepared as described above.

A polymer (resin) stock solution is prepared as follows: In a jacketedkettle kept at 40° C. using a circulating bath, 20% (w/w) PES resin(BASF, Ultrason E 6020 P), 20% (w/w) NMP, 59% (w/w) DMF, <1% PEG-4000,and <1% poly(2-vinylpyridine) (Scientific Polymer Products) are mixed at800 rpm using an overhead mixer for 4 hr. The solution is placed undervacuum at 200 mbar for 30 minutes to deaerate the solution.

A casting solution is prepared as follows: The resin stock solution andthe particle stock solution are combined in a flask and mixed at 30,000rpm for 2 min with final concentrations of 28% (w/w) particles, 10% PES,10% NMP, 52% DMF, <1% PEG-4000, and <1% poly(2-vinylpyridine), followedby deaeration at 200 mbar for 30 min.

Using a casting bar gapped to 5 mil, PVOH stock solution is cast onto aglass plate and placed in an oven at 80° C. for 2 hr. to form a film.Subsequently, the casting solution is cast onto the PVOH film using a 7mil gapped casting bar and placed in an oven for 15 min at 60° C.followed by immersion in water at 80° C. for 1 hr. to release themembrane from the coated substrate. The membrane is soaked in 0.5 mol/LHCl for 30 min., then 2 mol/L KOH for 4 hr. The membrane is subsequentlywashed with water at 25° C. for 2 hr and dried at 70° C. for 30 min.

After exposing the membrane to Ponceau S, a prominent red color isshown, thus showing the membrane has a positive charge at the surfaceand through the bulk.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A microporous membrane comprising a single layer having (a) a firstmicroporous surface; (b) a second microporous surface; and, (c) a firstporous bulk between the first microporous surface and the secondmicroporous surface, wherein the bulk comprises a first set of poreshaving outer rims, prepared by removing introduced dissolvable silicananoparticles, the first set of pores having a first controlled poresize, and a second set of pores connecting the outer rims of the firstset of pores, the second set of pores having a second controlled poresize, and a polymer matrix supporting the first set of pores, whereinthe first controlled pore size is greater than the second controlledpore size, and wherein the first and/or second microporous surfacecomprises a neutrally charged, a negatively charged, or a positivelycharged, surface.
 2. The microporous membrane according to claim 1,wherein the first set of pores have a controlled pore size in the rangeof from about 50 nm to about 1000 nm.
 3. The microporous membraneaccording to claim 1, wherein the first porous bulk comprises aneutrally charged, a negatively charged, or a positively charged, bulk.4. The microporous membrane of claim 1, wherein the membrane furthercomprises a second porous bulk comprising a third set of pores havingouter rims, prepared by removing introduced dissolvable silicananoparticles, the third set of pores having a third controlled poresize, and a fourth set of pores connecting the outer rims of the thirdset of pores, the fourth set of pores having a fourth controlled poresize, and a second polymer matrix supporting the third set of pores,wherein the third controlled pore size is greater than the fourthcontrolled pore size.
 5. The microporous membrane according to claim 4,wherein, (i) when the first porous bulk comprises a negatively chargedbulk, the second bulk comprises a neutrally charged, or positivelycharged, bulk; (ii) when the first porous bulk comprises a positivelycharged bulk, the second bulk comprises a neutrally charged, ornegatively charged, bulk; and (iii) when the first porous bulk comprisesa neutrally charged bulk, the second bulk comprises a negativelycharged, or positively charged, bulk.
 6. The microporous membraneaccording to claim 4, wherein the first set of pores and the third setof pores have substantially the same controlled pore size.
 7. Themembrane of claim 4, further comprising at least a third porous bulkcomprising a fifth set of pores having outer rims, prepared by removingintroduced dissolvable silica nanoparticles, the fifth set of poreshaving a fifth controlled pore size, and a sixth set of pores connectingthe outer rims of the fifth set of pores, the sixth set of pores havinga sixth controlled pore size, and a third polymer matrix supporting thefifth set of pores, wherein the fifth controlled pore size is greaterthan the sixth controlled pore size, the third porous bulk comprising adifferent charge than the first porous bulk or the second porous bulk.8. The membrane according to claim 1, further comprising an additionallayer having (i) a first porous surface; (ii) a second porous surface;and, (iii) a porous bulk between the first porous surface and the secondporous surface of the additional layer, wherein the porous bulkcomprises a fibrous matrix; Or (iv) a first microporous surface; (v) asecond microporous surface; and, (vi) a porous bulk between the firstmicroporous surface and the second microporous surface of the additionallayer, wherein the second porous bulk comprises: (a) a first set ofpores having outer rims, prepared by removing introduced dissolvablesilica nanoparticles, the first set of pores having a first controlledpore size, and a second set of pores connecting the outer rims of thefirst set of pores, the second set of pores having a second controlledpore size, and a polymer matrix supporting the first set of pores,wherein the first controlled pore size is greater than the secondcontrolled pore size; (b) a set of pores prepared by phase inversion,the set of pores having a controlled pore size; or (c) a fibrous matrix;or, (d) a set of pores prepared by stretching or track etching ore-beam, the set of pores having a controlled pore size.
 9. A method ofmaking a membrane, the method comprising: (a) casting a solutioncomprising a dissolvable silica nanoparticle-containing polymer solutioncomprising an uncharged polymer or a charged polymer onto a substrate;(b) carrying out phase inversion of the nanoparticle-containing polymersolution to provide a membrane; (c) dissolving the nanoparticles andobtaining a nanoparticle-depleted membrane comprising a neutrallycharged, a negatively charged, or a positively charged surface; and (d)washing the nanoparticle-depleted membrane.
 10. The method of claim 9,wherein (a) comprises casting the solution on a substrate pretreatedwith a preconditioning agent or a release agent.
 11. The method of claim10, wherein the preconditioning agent or the release agent is dried onthe substrate before casting the solution on the pretreated substrate.12. The method of claim 9, wherein (b) comprises exposing thenanoparticle-containing polymer solution to a temperature in the rangeof from about 40° C. to about 80° C. for a period in the range of fromabout 1 minute to about 2 hours.
 13. A method of filtering a fluid, themethod comprising passing the fluid through the membrane of claim
 1. 14.The microporous membrane according to claim 2, wherein the first porousbulk comprises a neutrally charged, a negatively charged, or apositively charged, bulk.
 15. The microporous membrane of claim 14,wherein the membrane further comprises a second porous bulk comprising athird set of pores having outer rims, prepared by removing introduceddissolvable silica nanoparticles, the third set of pores having a thirdcontrolled pore size, and a fourth set of pores connecting the outerrims of the third set of pores, the fourth set of pores having a fourthcontrolled pore size, and a second polymer matrix supporting the thirdset of pores, wherein the third controlled pore size is greater than thefourth controlled pore size.
 16. The microporous membrane of claim 2,wherein the membrane further comprises a second porous bulk comprising athird set of pores having outer rims, prepared by removing introduceddissolvable silica nanoparticles, the third set of pores having a thirdcontrolled pore size, and a fourth set of pores connecting the outerrims of the third set of pores, the fourth set of pores having a fourthcontrolled pore size, and a second polymer matrix supporting the thirdset of pores, wherein the third controlled pore size is greater than thefourth controlled pore size.
 17. The microporous membrane of claim 3,wherein the membrane further comprises a second porous bulk comprising athird set of pores having outer rims, prepared by removing introduceddissolvable silica nanoparticles, the third set of pores having a thirdcontrolled pore size, and a fourth set of pores connecting the outerrims of the third set of pores, the fourth set of pores having a fourthcontrolled pore size, and a second polymer matrix supporting the thirdset of pores, wherein the third controlled pore size is greater than thefourth controlled pore size.
 18. The microporous membrane according toclaim 5, wherein the first set of pores and the third set of pores havesubstantially the same controlled pore size.
 19. The membrane of claim18, further comprising at least a third porous bulk comprising a fifthset of pores having outer rims, prepared by removing introduceddissolvable silica nanoparticles, the fifth set of pores having a fifthcontrolled pore size, and a sixth set of pores connecting the outer rimsof the fifth set of pores, the sixth set of pores having a sixthcontrolled pore size, and a third polymer matrix supporting the fifthset of pores, wherein the fifth controlled pore size is greater than thesixth controlled pore size, the third porous bulk comprising a differentcharge than the first porous bulk or the second porous bulk.
 20. Themembrane of claim 5, further comprising at least a third porous bulkcomprising a fifth set of pores having outer rims, prepared by removingintroduced dissolvable silica nanoparticles, the fifth set of poreshaving a fifth controlled pore size, and a sixth set of pores connectingthe outer rims of the fifth set of pores, the sixth set of pores havinga sixth controlled pore size, and a third polymer matrix supporting thefifth set of pores, wherein the fifth controlled pore size is greaterthan the sixth controlled pore size, the third porous bulk comprising adifferent charge than the first porous bulk or the second porous bulk.